The present invention relates to an installation for storing of natural gas at high pressures of between about 3 and about 25 MPa and temperatures of from about xe2x88x9230xc2x0 C. to about 60xc2x0 C., in a lined underground storage space.
A rock cavity for storing of natural gas under pressure must fulfil a number of general criteria in respect of the pressure-absorbing capacity; among other things, the rock cover above the storage space must be capable of preventing up-lift or elevation, and the deformations in the surrounding rock must be limited when subjected to the existing pressure, such that they do not result in the elongation capacity of the impermeable lining layer being exceeded. In addition, it must be possible to drain groundwater from the surrounding rock to prevent damage to the impermeable lining layer of the storage space, said impermeable lining layer in most cases being made of steel plate.
The vital part of a lined gas storage space is a sandwich element which comprises the impermeable lining layer (in most cases steel plate, but in some cases also plastic), the concrete layer and the surrounding rock. Each of these elements has a function, i.e. the impermeable lining layer has the gas-sealing function, the concrete casing has the pressure-transmitting function and, finally, the rock has the pressure-absorbing function. At the same time the three elements must work together although it is a matter of a complex interaction between the structural parts when the system is pressurised. It is in fact important for the sandwich construction to retain its impermeability also after being subjected to pressure. The application of pressure leads to cracking in the concrete layer and deformation of the impermeable lining layer. The construction must, however, counteract the development of cracks in the impermeable lining layer. Furthermore the construction should be such as to allow draining of groundwater.
When storing pressurised natural gas in rock cavities, an impermeable lining layer of steel plate has been suggested in most cases. In this connection, use has been made of either steel plate or steel sheet. In a lined rock cavity concept, a pressure of up to 20 MPa has frequently been used.
To reduce the risk of leakage in the rock mass, a drainage layer has sometimes been arranged between the impermeable lining layer and the rock wall. WO 90/08241 describes this known technique. According to this publication, a gas storage tank has an inner shell of steel plate supported by a layer of corrugated steel sheet. The latter layer is anchored to the rock wall by means of rock bolts and has inwardly directed fixing projections that serve as attachments for the impermeable lining layer so as to prevent it from moving relative to the layer of corrugated steel sheet.
WO 86/01559 discloses one more known technique for lining a gas storage rock cavity. As impermeable lining layer, use is made of an inner shell made of steel plate, surrounded and supported by annular connecting elements. These are anchored to the rock wall and also to the steel plate inner shell. Between the inner shell and the rock wall there is a filling material which in some embodiments consists of on-site cast concrete.
A similar technique is disclosed in EP-B-0 153 550, which concerns a method of building large cylindrical storage tanks with concentric plate walls, of which the outer abuts against a prestressed concrete wall and between which an insulating layer is arranged.
EP-A-0 005 133 also discloses a gas storage tank which is placed in a rock cavity shaft and stands freely in this shaft. In one embodiment, the shaft is provided with a lining of water-tight concrete and a cast concrete floor.
EP-A-0 401 154 discloses a free-standing gas storage tank with a gas-tight inner wall made of steel plate material and a force-absorbing outer wall made of prestressed concrete, an insulating layer being arranged between the inner wall and the outer wall.
WO 85/04214 also discloses a rock cavity storage space for storing, inter alia, gases. In this case, the rock wall has first been provided with a water-permeable layer of sprayed concrete, which serves as a drainage layer for groundwater and which constitutes a supporting surface for an inner lining of, for instance, concrete and steel plate.
WO 89/02864 discloses a rock cistern for pressurised storing of natural gases. The walls of the cistern comprise a sprayed concrete layer next to the rock wall which is reinforced through anchoring bolts. The concrete layer is covered by a sealing comprising one reinforced sealing layer and one unreinforced plastics layer. The layers are bonded to the rock surface by means of a binding layer which serves to transfer stresses, in the form of limited shear stresses, between the sprayed concrete and the overlaying sealing layer.
GB-A-2 215 023 discloses an underground fluid storage cavern for storing low pressures of 0.1 to 5 MPa at temperatures of xe2x88x9280 to xe2x88x92170xc2x0 C. The cavern wall comprises a tight inner layer and a porous concrete layer between the rock surface and the tight inner layer. Compressed air is fed into this porous concrete layer to maintain the air pressure at a pressure greater than the ground water pressure.
U.S. Pat. No. 3,683,628 discloses a reservoir for storing fluids in underground cavities. In this case sheets of impermeable plastic material are secured placed between a concrete layer and a fluid tight flexible liner. This reservoir is not suited for the storing of natural gas at high pressures of between about 3 and about 25 MPa and temperatures of between about xe2x88x9220xc2x0 C. and 60xc2x0 C.
GB-A-493 893 discloses a cistern or reservoir for the storage of hydrocarbon fluids at about atmospheric pressure. The cistern wall comprises several layers which are separated by and bonded to each other by means of a plastics layer. This construction is not suited for the storage of natural gas at the pressures and temperatures contemplated herein.
WO 87/00151 discloses rock cavern having a wall comprising a drainage layer and a concrete layer next to the rock and a sealing layer next to the inside of the cavern. The concrete layer is bolted to the rock. The sealing layer consists of thin steel sheet strips welded together and spot-welded to a strip steel attached to the concrete layer.
The known embodiments of rock cavity storage spaces for storing pressurised gas have various drawbacks, inter alia, that there is still a great risk of cracking in the impermeable lining layer and harmful cracking in the concrete layer supporting the impermeable lining layer.
One object of the present invention therefore is to provide a gas storage installation, in which these and other drawbacks are eliminated or considerably reduced.
This and other objects of the invention are achieved by a storage installation as described herein below.
In summary, the invention is related to an installation for storing of natural gas or some other fluid, e.g. compressed air. This installation comprises a lined underground storage space. Inside the rock wall of the storage space there is a concrete layer for supporting an inner impermeable lining layer. According to the invention, there is between the impermeable lining layer and the concrete layer a non-binding sliding layer to facilitate relative movements between the impermeable lining layer and the concrete layer. The concrete layer has a crack distribution control reinforcement layer closer to the impermeable lining layer than to the rock wall. The basic idea of the invention is not to completely avoid the propagation of cracks from the rock through the concrete layers to the impermeable lining layer but to intentionally distribute the cracks over a larger area by dividing large cracks into several smaller cracks, thus distributing shear forces acting on the impermeable lining layer over a larger area thereof.
Such a distribution of shear forces decreases the risk of deforming the lining layer to such a degree that cracks develop in the impermeable lining layer also when the impermeable lining layer is subjected to cyclic deformation during use of the cavern.